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Dewatering in biological wastewater treatment: A

review

Morten Lykkegaard Christensen* , Kristian Keiding, Per Halkjær Nielsen,Mads Koustrup Jørgensen

Department of Chemistry and Bioscience, Aalborg University, Frederiks Bajers Vej 7H, DK-9220 Aalborg East,

Denmark

a r t i c l e i n f o

Article history:

Received 2 February 2015

Received in revised form

15 April 2015

Accepted 17 April 2015

Available online 26 April 2015

Keywords:

Consolidation

Filtration

Resistance

Activated sludge

Pumping 

a b s t r a c t

Biological wastewater treatment removes organic materials, nitrogen, and phosphorus

from wastewater using microbial biomass (activated sludge, biofilm, granules) which is

separated from the liquid in a clarifier or by a membrane. Part of this biomass (excess

sludge) is transported to digesters for bioenergy production and then dewatered, it is

dewatered directly, often by using belt filters or decanter centrifuges before further

handling, or it is dewatered by sludge mineralization beds. Sludge is generally difficult to

dewater, but great variations in dewaterability are observed for sludges from different

wastewater treatment plants as a consequence of differences in plant design and physical-

chemical factors. This review gives an overview of key parameters affecting sludge dew-

atering, i.e. filtration and consolidation. The best dewaterability is observed for activated

sludge that contains strong, compact flocs without single cells and dissolved extracellular

polymeric substances. Polyvalent ions such as calcium ions improve floc strength and

dewaterability, whereas sodium ions (e.g. from road salt, sea water intrusion, and industry)

reduce dewaterability because flocs disintegrate at high conductivity. Dewaterability

dramatically decreases at high pH due to floc disintegration. Storage under anaerobic

conditions lowers dewaterability. High shear levels destroy the flocs and reduce dew-

aterability. Thus, pumping and mixing should be gentle and in pipes without sharp bends.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Municipal and industrial wastewater contain high amounts of 

COD, nitrogen, and phosphorus, which are usually degraded

or removed by biological wastewater treatment (Lindrea and

Seviour, 2002). The activated sludge process is by far the

most common process, but alternative processes such as

biofilm systems or granules systems also exist (de Bruin et al.,

2004). An integrated part of the biological wastewater treat-ment is thesolideliquidseparation, where the treated water is

separated from the activated sludge. In the conventional

activated sludge process, this is done by clarifiers, but there is

an alternative: membrane bioreactors, where a membrane is

used instead of the clarifier (Brindle and Stephenson, 1996;

Lindrea and Seviour, 2002). The outcome of the process is

treated wastewater (effluent), return sludge, and excess

sludge.

*   Corresponding author. Tel.: þ45 9940 8464.E-mail address: mlc@bio.aau.dk (M.L. Christensen).

 Available online at www.sciencedirect.com

ScienceDirect 

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In some cases, excess sludge is transported to digesters for

sludge reduction and bioenergy production. However, in

many cases, other types of sludge handling takes place, e.g.

transportation to agricultural fields or drying and incinera-

tion. Since the water content of excess sludge is high, it mustbe dew before further handling, typically by belt filters, filter

press, decanter centrifuges, and sludge mineralization beds

(Sørensen and Sørensen, 1997). Thus, several solideliquid

separation processes are involved in wastewater treatment

for separating sludge from the treated wastewater as well as

for sludge dewatering. The dewatering process is costly, and

the composition and properties of the sludge are important for

the separation process (Bruus et al., 1992; Sørensen and

Sørensen, 1997; Chu et al., 2005).

This paper reviews the existing literature on sludge dew-

aterability, i.e. sludge filtration and consolidation.   Fig. 1

summarizes the key parameters that affect various sludge

properties such ad dewaterability. Sludge contains flocs, andsludge properties are mainly determined by the size, shape,

density and strength of the sludge flocs. Thus, an under-

standing of the sludge flocs is crucial for a more general un-

derstanding of sludge dewatering. Flocs, on the other hand,

consist of microorganisms, extracellular polymeric sub-

stances (EPS), organic debris and inorganic particles. Some of 

the components are produced during the biological process

and some of the components come from the influent. Further,

floc density and strength are influenced the content of e.g.

catons and inorganic particle and also by shear forces and

thereby indirectly by the design and operation of the plant.

The floc properties not only influence sludge filtration and

consolidation but also other processes such as flocculation,settling and membrane fouling, i.e, literature data show that

sludge components that cause problems in filtration and

consolidation also cause problems in other types of separation

processes (e.g. sedimentation, centrifugation, sludge miner-

alization bed, and membrane bioreactors). Thus, many of the

conclusions from this paper are of generic value for all solid-

eliquid separation processes for biological sludges.

2. Sludge composition

Biological activated sludge consists primarily of biological

flocs that are formed by growth of microorganisms and by

adsorption of particles from the influent. The flocs consist of 

microorganisms, either as single cells, filamentous bacteria or

microcolonies, organic fibers, inorganic particles (salt and

sand), and extracellular polymeric substances (EPS). The

typical size of theflocsis 129± 109 mm (Mikkelsen and Keiding,2002) e  see sketch of a typical sludge floc in Fig. 2.

Sludge flocs have a fractal-like structure and are kept

together by DLVO forces (van der Waals and electrostatic

forces), non-DLVO-forces (bridging, hydrophobic forces), and

physical entanglement (Namer and Ganczarczyk, 1994;

Cousin and Ganczarczyk 1999; Nielsen, 2002). EPS compo-

nents are particularly important for the floc properties. The

EPS components are a mixture of different macromolecules,

e.g. proteins, humic-like substances, polysaccharides, nucleic

acids and lipids and contribute with 40e60% of the total dry

matter of the flocs (Nielsen, 2002). They are negatively

charged, and the charge density has been measured to be

0.2e

1 meq/g EPS (Keiding et al., 2001; Mikkelsen and Keiding,2002; Reynaud et al., 2012). Different methods exist for EPS

extraction and analyses, and it is often difficult to compare

literature data. Nevertheless, it is generally accepted that EPS

can be classified as tightly bound EPS (TBEPS), loosely bound

EPS (LBEPS), and suspended EPS. Further, a dynamic equilib-

rium has often been found between loosely bound and sus-

pended EPS components (Nielsen and Jahn, 1999; Comte et al.,

2006; Dominguez et al., 2010). The electrostatic interaction

Fig. 1  e  Overview of parameters that directly or indirectly influence sludge properties.

Fig. 2  e  Schematic picture of activated sludge flocs (the

ideal floc) from Nielsen et al. (2012).

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plays an important role for the equilibrium, and for this

reason, the concentration and valence of ions play a major

role for the floc structure, which will be discussed later in

more detail.

Sludge flocs contain large amounts of water; the reported

water content varies from 63% to 99% (Andreadakis, 1993;

Chung and Lee, 2003; Vaxelaire and Cezac, 2004). The water

in the flocs and at the surface of the flocs is often denotedbound water as opposed to thefree water,whichis notaffected

by the solid particles (Vesilind, 1994). Further, bound water has

been divided into three types of water pools: I) water trapped

insidecrevicesand the interstitialspace of the flocs (interstitial

water), II) water physically bound to surfaces (vicinal water),

and III) water chemically bound to solid materials (water of 

hydration). Alternatively, the high water content in flocs has

been explained as a consequence of the colligative properties,

i.e. the reduced water activity in the flocinteriordue to counter

ions (osmotic water) (Keiding et al., 2001).   Mikkelsen and

Keiding (2002) use the term   “water-holding ”   for the surface

bound water, the osmostic water, and the trapped water.

However, in both cases, the flocs contain water, some of whichis removed during compression, i.e. according to Deng et al.

(2011), interstitial water accounts for more than 50% of the

waterin the flocs and is at least partly removed by mechanical

dewatering (Novak, 2006). The dewatering process therefore

depends on the strength of the floc structure.

The floc structure can vary from large compact flocs (the

ideal floc), flocs with high abundance of filamentous bacteria

(filamentous bulking), or small, light flocs without filamentous

bacteria (pinpoint floc). In rare cases, no or few flocs are

formed with many single bacteria (dispersed growth). Gener-

ally, the best separation properties are obtained if the sludge

contains large compact flocs, few filamentous bacteria

and few single cells (Bruus et al., 1992; Rasmussen et al., 1994).This gives the best settling in the clarifier, the highest

permeate flux in MBR systems (lowest fouling), the highest

filterability(belt filters and sludge mineralization bed), andthe

best effluent quality (decanter centrifuges) and lowers the

amount of chemicals required for sludge conditioning (Lee

et al., 2003; Masse et al., 2006; Dominiak et al., 2011a; Bugge

et al., 2013). However, there are some important differences

between the separation processes. The filamentous bacteria,

for example, are important for settling and sludge minerali-

zation (drainage) but not filtration and consolidation where

higher pressures is applied for compression (Dominiak et al.,

2011a; Bugge et al., 2013).

The species composition of the activated sludge influencesthe floc properties to a certain extent and thus the solid-

eliquid separation processes (Nielsen et al., 2002, 2004;

Klausen et al., 2004; Larsen et al., 2006, 2008; Bugge et al.,

2013). Some species form filaments, some strong micro-

colonies, and some weak flocs. They also produce different

amounts and type of EPS with different water-binding prop-

erties. The variation observed in solideliquid separation pro-

cesses in different treatment plants (see later) is therefore

caused by variations in both microbial composition and

water/floc chemistry. Recent studies by molecular DNA-based

methods have furthermore revealed that, despite presence of 

numerous bacterial species in the wastewater treatment

plants, the dominant and abundant ones can be found among 

only approx. 150 species that are present in most plants

(called core species) (Nielsen et al., 2010, 2012). These are now

studied in great detail to understand their identity, physi-

ology, ecology, impact on floc properties, and their possibil-

ities of manipulation of the community composition and

design of good solideliquid separation processes (McIlroy

et al., 2015; see for example the open resource   http://

midasfieldguide.org/).

3. Specific filtration flow rate

Several methods exist for comparing dewaterability of 

different types of sludge such as capillary suction time, sludge

volume index, average specific resistance of the cake, and the

specific filtration flow rate. The specific filtration flow rate

(SFF) is a useful term especially for filtration and consolidation

processes. Dewatering often involves both filtration (cake

formation) and consolidation (cake compression), and for

biological sludge it is difficult to distinguish between the two

processes (Stickland et al., 2005). Thus, the term dewater-ability is here used to describe the rate of both the filtration

and consolidation. When SFF is used, the dewaterability is

determined by the liquid flow through a cake consisting of the

solid materials from the suspension.

The liquid flow through a cake structure can be calculated

by using Eq. (1) if the filter medium resistance is low:

q ¼  p

maavuc(1)

where   q   [m3 /(m2s)] is the filtrate flux,   m   (Pa s) is the filtrate

viscosity, p (Pa) is the filtration pressure,uc (kg/m2) isthe mass

of solid materials per unit filter media area, and  aav (m/kg) is

the average specific resistance of the cake.The average specific resistance is independent of filtration

pressure for incompressible cakes. However, most cakes are

compressible, i.e. cake porosity decreases with increasing 

pressure, whereby the average specific resistance increases as

well. There exist several constitutive equations describing this

relationship between average specific resistance and pres-

sure, one such equation has been suggested by Tiller and Yeh

(1987):

aav  ¼ a0

1 þ

  p

 ps

n

(2)

where   a0   (m/kg) is the average specific resistance at zero

pressure, and  ps  (Pa) and  n   () are empirical constants. Theconstitutive equation was originally developed for the local

specific cake resistance, but is also applicable as an empirical

equation for calculating the average specific cake resistance.

For sludge, the average specific resistance usually in-

creases almost linearly with pressure (Sørensen and

Sørensen, 1997), i.e.  n  ¼  1 and  ps ≪  p. Thus, due to the high

compressibility, it is necessary to know the filtration pressure

in order to compare measured literature values of average

specific resistances. However, there exists an alternative and

more useful way to characterize the liquid flow through a cake

with high compressibility, the specific filtrate flow rate (SFF).

This has been defined in Sørensen et al. (1996)  and Sørensen

and Sørensen (1997):

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SFF ¼ quc  ¼  p

maav(3)

The permeate flux is inversely proportional to cake thick-

ness, so the product of flux and cake thickness (SFF) does not

change during constant pressure filtrations. For non-

compressible cakes, SFF increases proportionally with

applied pressure, whereas the SFF value is constant and in-

dependent of the pressure for highly compressible cakes, as

an increase in pressure will be equalized by the subsequent

elevation of specific resistance. Thus, for biological sludge that

forms highly compressible cakes, SFF is the better measure of 

the dewaterability than e.g. the often used average specific

resistance.

In   Fig. 3, it is seen that SFF increases with pressure for

kaolin, whereas it is almost constant for biological sludge at

pressures above 2 kPa and equals 2.7$105 kg/(ms) (Fig. 3). The

data confirm that SFF for compressible cakes is independent

of pressure except at low pressure and can therefore be used

to compare data from different types of sludges.

The SFF has been measured for excess sludge from seven

different wastewater treatment plants with nutrient removal

in Denmark,and the data show that SFF varies by a factor of 10

for the different types of activated sludges (Fig. 4).   Table 1

shows the structure of the flocs for the filtered sludges. The

data confirm that large compact flocs give highest SFF and

thus the highest dewaterability.

The main conclusion is that sludge composition has a high

impact on sludge dewaterability. Several other studies

confirm that the dewaterability varies for different sludges

(Karr and Keinath, 1978; Katsiris and Kouzeli-Katsiri, 1987;

Novak et al., 1988; Cho et al., 2005; Cicek et al., 1999; How et al.,

2005). Thus, in order to understand how sludge properties

affect specific filtration flow rate, cake structure and

compression will be discussed in more details.

4. Sludge cake compressibility and blinding

When the sludge cake is compressible, it means that the cake

porosity,   ε, decreases with increasing pressure, resulting in

higher resistance. This can be modeled by the following 

constitutive equations (Tiller and Yeh, 1987):

ð1 εÞ ¼ ð1 ε0Þ

1 þ

 p

 ps

b

(4)

where b  is an empirical parameter and  ε0 is the porosity of an

uncompressed cake. The equation is similar to the one used

for the average specific cake resistance, and combining the

two equations givesaav¼ k(1ε)m, where k is the ratio between

a0 and (1ε0), and m is the ratio between n  and  b.Eq.   (4)   gives the cake porosity and thereby the final dry

matter content of the cake, which for compressible cakes in-

creases with applied pressure.

Several mechanisms have been suggested to explain the

porosity reduction. These mechanisms are summarized in

Table 2 and Fig. 5.

Many studies have focused on the filtration of inorganic

particles, where the compressibility is generally well

described and understood. Large inorganic particles (>10  mm)

usually form incompressible cakes, and the porosity of the

cake is mainly dependent on particle structure (Tiller and Yeh,

1987). Cakes consisting of colloidal particles may be

compressible, depending on the degree of flocculation, i.e.highly flocculated colloidal particles form cakes with high

porosity and high compressibility (Tiller and Yeh, 1987). The

degree of flocculation depends on particle surface properties

(mainly charge density) and physico-chemical properties of 

the suspension, e.g. ionic strength. During consolidation

(compression) of inorganic cakes, two consolidation stages

have been observed, of which one has been ascribed to the

collapse of the global cake structure (Point 1,  Table 2), and one

ascribed to particle migration into a more stable configuration

(Point 2, Table 2) (Shirato et al., 1986; Chu and Lee, 1999; Xu

et al., 2004). The global cake collapse is controlled by the hy-

draulic resistance of the cake (Shirato et al., 1986). Particle

migration is controlled by the highly viscous surface-absorbed

Fig. 3 e  Specific filtrate flow rate for kaolin and excess

sludge, recalculated from Sørensen and Sørensen (1997).

Fig. 4 e Specific filtrate flow rate for activated sludge from 7

different full-scale wastewater treatment plants in

Denmark, recalculated from Dominiak et al. (2011a).

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water between the particles and is a slow process, compared

to the overall collapse of cake structure (Chu and Lee, 1999).

Compression of cakes consisting of colloidal particles is also

partly due to the reduction of the distance between the par-

ticles (Point 3, Table 2) (Koenders and Wakeman, 1997; Keiding 

and Rasmussen, 2003). The distance between particles is a

function of the electrostatic repulsion, van der Waal attrac-

tion, and the external pressure (Koenders and Wakeman,

1997).

For biological sludges, the compressibility is less well

described and understood, but it is believed that the same

mechanisms are relevant as for inorganic sludge (Chu and

Lee, 1999). Furthermore, organic sludge consists of soft

water-swollen materials; thus, deformation and compression

of individual particles are important as well (Point 4,  Table 2).

The effect of compression of water-swollen particles has been

investigated by synthesizing and filtering model particles. A

study of synthetic polystyrene-co-poly(acrylic acid) shows

that the soft polyacrylic acid shell deforms and compresses

during filtration, which lowers the specific flow rate by a factor

of 10e100 because the soft materials fill out the void between

the particles (Lorenzen et al., 2014). The specific flow rate is

low, compared to inorganic particles of the same particle size,

but the SFF values measured are comparable with values

found for biological sludge (Lorenzen et al., 2014). Further-

more, a relatively large reduction of the cake water content is

observed during consolidation of both sludge and the syn-

thetic polystyrene-co-poly(acrylic acid) particles (Christensen

and Keiding, 2007; Christensen and Hinge, 2008). Hence,

compression and deformation of individual particles explain

the high compressibility for sludge (Hwang and Hsueh, 2003).

For suspensions containing particles with large particle

size distribution, small particles may be trapped within the

cake pores, often denoted cake blinding (Point 5,   Table 2)

(Christensen and Dick, 1985; Sørensen et al., 1995). Sometimes

filtration cannot be described using the traditional filtration

theory. Such data are observed for sludge cakes and has been

explained as an effect of cake blinding, i.e. small particles

seem to be particularly important for the dewaterability of 

sludge. Cake blinding can also be observed due to floc dis-

rupture and erosion (Sørensen et al., 1995). Thus, not only

particle size, degree of aggregation and structure are impor-

tant for dewaterability of sludge, but also the presence of 

small particles, water content of the flocs, and floc strength.

Several factors influence floc properties, concentration of 

single cells, and suspended EPS. The physico-chemical prop-

erties of the suspension are important for the sludge proper-

ties and especially the ions in the solution.

5. Conductivity and water hardness

The composition of inlet wastewater varies from plant toplant, e.g. due to different industries, rainfall, etc. This affects

both the biological wastewater treatment and the dewater-

ability of the biological sludge produced. Several studies have

shown that the wastewater conductivity, water hardness, and

pH vary; i.e. ionic composition and concentrations vary. This

strongly affects the dewaterability of the biological sludge.

Both floc structure and strength strongly depend on ionic

composition and concentration. High concentrations of 

multivalent cations, such as calcium and magnesium, give

strong and compact flocs (Biggs et al., 2001; Higgins et al.,

2004a; Larsen et al., 2008). Data show that the porosity of the

flocs decreases with increasing calcium ion concentration

(Cousin and Ganczarczyk, 1999). Conversely, monovalentcations such as sodium and potassium lower floc strength

(Higgins and Novak, 1997; Biggs et al., 2001). Different theories

have been suggested to explain the role of cations in sludge

flocculation, e.g. the alginate egg-box model, the DLVO theory,

and divalent cation bridging, of which the divalent cation

bridging model seems to describe the role of ions in sludge

best (Sobeck and Higgins, 2002; Higgins et al., 2004a). Accord-

ing to the divalent bridging model, calcium and other divalent

ions bridge the negatively charged sites on EPS and thereby

form a matrix of EPS and single cells. Several studies have

confirmed the positive effect that divalent ions have on floc

structure and dewaterability. Activated sludge samples from

different membrane bioreactor plants show that an increased

Table 1 e  Sludge and floc properties ( Dominiak et al., 2011a ).

Plant Microscope analysis Relative SFFa

Bramming South Large compact, round, dark flocs 100

Esbjerg West Large, regular compact flocs 38

Hjørring Medium-sized flocs, both round, regular and open irregular 24

Esbjerg East Open irregular, medium-sized flocs 21

Aalborg East Medium-sized flocs, both compact and open 16

Bramming North Very small, irregular, disintegrated flocs, many branched filamentous

bacteria

12

Aalborg West Small, irregular flocs of open structure 12

a Relative SFF setting Bramming South to 100.

Table 2 e  Cake structure phenomena in filtrations.

1 Collapse of global structure and dissipation of 

excess pore water. This includes bending and

slipping of fibers.

2 Particle migration into a more stable

configuration.

3 Reduction of inter-particular distance.

4 Deformation and compression of individu al

particles.

5 Cake blinding from small particles from the

suspension (a) or from disintegration of flocs or

individual particles in the cake (b).

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ratio between divalent ions (calcium/iron) and EPS correlates

with lower amounts of single cells in the bulk solution, in-

creases the flocs sizes as well as the dewaterability (Bugge

et al., 2013). Bruus et al. (1992) added EGTA to sludge in orderto remove calcium ions from sludge flocs, resulting in desta-

bilization of the flocs, increase of the concentration of single

cells and soluble EPS, and thereby a reduction of the dew-

aterability. Peeters et al. (2011) showed that the exchangeable

calcium fraction is about 0.7 meq/g MLVSS, whereas calcium

is precipitated as calcium salts (e.g. calcium carbonate) within

the floc structure at higher calcium concentrations.

At high concentrations of monovalent ions, the divalent

ions in the floc matrix are ion exchanged by monovalent ions,

which weakens the floc structure. Bruus et al. (1992) showed

that the concentration of calcium in the bulk increases after

addition of monovalent salts, whereby the concentration of 

single cells increases,and the dewaterability drops. Due to the

ion exchange between mono- and divalent ions, the ratio be-

tween monovalent (Mþ) and divalent cations (Dþþ) should,

according to Higgins and Novak (1997), be lower than 2 on a

meq/L basis to ensure good dewaterability. Others suggest

that good sludge dewaterability is observed as long as Mþ /

Dþþ < 4 (Peeters et al., 2011).

The literature therefore shows that water hardness (con-

centration of multivalent ions) is important for sludge dew-aterability. Furthermore, addition of calcium ions will usually

improve sludge dewaterability, and due to the beneficial effect

of divalent ions, the literature has suggested to use alterna-

tives to sodium-based chemicals, i.e. chemicals containing 

divalent cations instead of sodium (Higgins et al., 2004b). High

conductivity due to monovalent ions reduces dewaterability,

and this phenomenon can be observed in the northern part of 

Europe due to road salting during winter and intrusion of sea

water. Moreover, wastewater from some types of industries

has high conductivity. The typical conductivity of Danish

activated sludge is 750   mS/cm, but values up to 4400   mS/cm

have been observed (PH Nielsen, unpublished).

6. Sludge pH

Activated sludge flocs contain a lot of EPS which contain

titratable groups and are negatively charged at neutral pH.

The EPS components are almost non-charged at pH around

2.6e3.6 (Liao et al., 2002), whereas the charge increases with

pH (Raynaud et al., 2012). As EPS components and electrostatic

forces play a central role in floc structure, sludge pH indirectly

affects the floc structure and sludge dewaterability. At low pH,

the bulk suspension only contains few colloidal particles, and

the dewaterability of sludge is generally high (Karr and

Keinath, 1978). At high pH, the number of colloidal particlesand suspended EPS increases (floc disintegration), and the

dewaterability drops (Karr and Keinath, 1978; Raynaud et al.,

2012). By using data from   Raynaud et al. (2012), it can be

shown that the SFF values decrease from 4.4$105 kg/(ms) at

pH 7 to 0.9$105 kg/(ms) at pH 9, which is a reduction of 

approximately 80%. The effect of adding acid (lowering pH) is

not as pronounced as adding base (increasing pH).  Raynaud

et al. (2012) observed a small reduction in dewaterability by

reducing pH to 3, where SFF was reduced to 4.2$105 kg/(ms),

whereas   Karr and Keinath (1978)  observed higher dewater-

ability after addition of acid (pH ¼ 3). Liao et al. (2002) did not

observe any change in dewaterability at low pH. However, the

water content was reduced in the formed cake if the pH waslowered before filtration. Thus, pH affects dewaterability, and

high pH value should be avoided.

7. Biological process

The solideliquid characteristics of the sludge is influenced by

the wastewater composition and the way the sludge is pro-

duced, e.g. by the conventional activated sludge process,

membrane filtration in MBR, biofilms, or by mesophilic and

thermophilic digestion.

Table 3  summarizes sludge characteristics and filtration

properties from two surveys of sludge filtration properties in

Fig. 5 e  Illustration of mechanisms of cake compression

and blinding described in Table 2.

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terms of total EPS content, mean floc size, shear sensitivity for

the release of particles during shear treatment, kSS, and SFF

(Mikkelsen and Keiding, 2002; Bugge et al., 2013).

The SFF of anaerobically digested sludge is low, compared

with other types of sludges. Anaerobically digested sludge has

low concentrations of EPS, compared with activated sludge,

which seems to correlate with smaller flocs. Furthermore, the

ionic strength and the number of single cells increase during anaerobic storage (Rasmussen et al., 1994). Sludge floc

strength significantly decreases for anaerobically digested

sludge (Table 3). Thus, floc deformation, compression, and

disintegration are more pronounced in dewatering of anaer-

obically stored or digested sludge.

The main difference between MBR and CAS system is the

use of a membrane instead of a clarifier. Generally, MBR

sludge has lower SFF, compared with CAS sludge (Cicek et al.,

1999; How et al., 2005). The low SFF of MBR sludge can be

ascribed to the low degree of flocculation, differences in

microbiology, and principle of separation (Masse et al., 2006,

Geng and Hall, 2007,  Lee et al., 2003, Van den Broeck et al.,

2010; Van den Broeck et al., 2012). In CAS treatment, there isa selection for flocculated bacteria, while dispersed bacteria

are removed with the effluent. For MBR sludge, dispersed

bacteria are too large to penetrate the pores of the membrane.

Therefore, MBR sludge shows a higher content of single cells

and soluble EPS, compared to CAS sludge (Wagner et al., 2000;

Witzig et al., 2002; Masse   et al., 2006; Merlo et al., 2004;

Sperandio et al., 2005), whereas CAS sludge has a higher

contentof EPS (Table 3), which is important for bioflocculation

(Merlo et al., 2004; Masse et al., 2006). Furthermore, the higher

level of shear in an MBR tank induced by e.g. air scouring of 

the membranes does not allow large flocs to form (Cicek et al.,

1999). Thus, thefloc size is usually lower in MBR sludge than in

other types of sludges. Lower floc sizes result in higher specificresistance to filtration due to cake blinding, smaller flocs, and

less permeable and more compressible cakes (Lee et al., 2003).

Thus, SFF can be a factor of three lower for MBR sludge than

CAS sludge (Cicek et al., 1999).

8. Sludge storage

Sludge is often stored before dewatering. However, the bio-

logical processes do not stop during storage; thus, floc

structure and composition change, which in turn affects and

often reduces dewaterability (Bruus et al., 1993). Several fac-

tors are involved in these changes such as hydrolysis of EPS

components, reduction of Fe(III) to Fe(II), which is a poorer

flocculant, and production of sulfide by microbial sulfate

reduction that subsequently precipitates and removes Fe (III)

and Fe(II) (Nielsen and Keiding, 1998; Wilen et al., 2000a,b). A

study of 10 days' anaerobic storage of sludge shows a signifi-cant increase in the number of single cells, conductivity, and

bulk calcium concentration (Rasmussen et al., 1994). As dis-

cussed in the previous section, this reduces dewaterability,

and in the cited study, anaerobic storage reduces the specific

flow rate by 80%. Mixing of anaerobically stored sludge further

lowers the SFF value (Parker et al., 1972; Larsen et al., 2006).

The negative effect of storage can be limited by ensuring 

aerobic or anoxic conditions during storage, e.g. by aeration or

addition of nitrate (Dominiak et al., 2011a). The reduced SFF

after anaerobic storage can to some extent be improved again

by aeration, i.e. ensuring aerobic storage (Parker et al., 1972;

Wilen et al., 2000b). The negative consequence of anaerobic

storage may also be important for the activated sludge pro-cess; there may exist anaerobic zones in the plants, which

impairs the sludge, causes deflocculation and thereby reduces

dewaterability of the biological sludge produced.

9. Pumping and stirring of sludge

Sludge flocs can be destroyed due to high shear levels which

reduce sludge dewaterability. Particles and sludge flocs aggre-

gate under low shear rates andbreakup at elevated shear rates

(Mikkelsen and Keiding, 1999, 2002). Break-up of sludge flocs

(fragmentation) lower the mean size of the flocs ( Jarvis et al.,

2005). At higher shear rates, smaller particles (e.g. single cells)

are desorbed from the floc surface due to erosion (Mikkelsen

and Keiding, 2002; Biggs et al., 2003). Both floc size and espe-

cially the number of single cells affect the dewaterability. The

specific flowrate is reduced after vigorousstirring of the sludge

(Dominiak et al., 2011b), as the increased numberof single cells

and lower particle sizes lead to cake blinding. The negative ef-

fect of high shear depends on the floc strength, i.e. the floc

resistance to stirring. It has been shown that calcium ions

reduce the effect of shear. Conversely, anaerobic storage results

inweakflocsthateasilybreakupduringhighshear(Rasmussen

Table 3 e  Physical-chemical characteristics of primary, activated, digester (mesophilic, thermophilic) feed with surplusactivated sludge and MBR sludge.

Activated sludgea MBR sludgeb Mesophilic sludgea Thermophilic sludgea

Total protein (mg/gSS) 346  ±  111 185  ±  45 248  ±  12 155  ±  62

Total humics (mg/gSS) 58  ±  35 22  ±  8 112  ±  108 188  ±  92

Total polysaccharides (mg/gSS) 101  ±  35 111  ±  13 70  ±  5 78  ±  10

EPS (mg/gSS) 130  ±  65 89  ±  11 78  ±  49 41  ±  9

Mean floc size (mm) 125  ±  109 65  ±  23 51  ±  21 57  ±  11

Shear sensitivity, kSS   0.062  ±  0.049 0.102  ±  0.066 0.244  ±  0.016 0.418  ±  0.337

SFF (kg/(m∙s)) 83.3$107 72.9$107 9.7$107 0.78$107

a Data from Mikkelsen and Keiding (2002).b

Data from Bugge et al. (2013).

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et al., 1994; Larsen et al., 2006, 2008). In general, viscous shear

shouldbe avoided in orderto ensurea high specificfiltrate flow,

e.g. by ensuring gentle pumping, gentle mixing, no storage in

tank or pipes, and sharp pipe bends should also be avoided.

10. Summery of factors that influence sludge

quality

Thus, several parameters affect the dewaterability of sludge:

the physico-chemical properties of the feed, the biological

treatment, and the handling of the sludge before and during 

dewatering. Table 4 summarize the conclusions from the text.

The composition of the incoming wastewater affects the

properties of the sludge produced, especially the organic

compounds, pH, and the ion composition. The biological

process and the plant design as well as the further sludge

handling (pumping, mixing, and storage) are important for the

sludge flocs and the dewaterability of sludge.

11. Improvement of sludge filterability byflocculation

Sludge dewatering is an expensive operation in wastewater

treatment plants. It is not possible to improve the dewatering 

process by applying higher pressure in filtration processes due

to the high compressibility of sludge cakes. Instead, sludge

can be pre-treated by adding coagulants e.g. polyaluminium

chloride (PAC) or ferric salts (FeSO4Cl), followed by addition of 

flocculants or by adding flocculants alone. This improves

sludge dewaterability significantly and reduces the costs of 

the separation process. It should be mentioned that addition

of inorganic salts may have negative effects on further sludge

handling e.g. if the sludge is incinerated or if phosphorus from

the sludge has to be reused as a fertilizer.

The degree of sludge flocculation is enhanced by addition

of coagulants and flocculants. Addition of e.g. ferric chloride,

and thereby positively charged multivalent metal ions (Fe3þ),

strengthens the floc structure and removes EPS and single

cells from the bulk (Poon and Chu, 1999; Wilen et al., 2008; Niu

et al., 2013). Multivalent cations adsorb to surfaces, which

reduces the electrostatic repulsion between negatively

charged particles, e.g. flocs and single cells, whereby they

aggregate (Niu et al., 2013).

Addition of polyvalent cationic polymers enhances floc-

culation by charge neutralization and polymer bridging 

(Bolton and Gregory, 2007). Data show that addition of floc-

culant with efficient mixing increases floc size, increases SFF,and lowers cake compressibility (Chu et al., 2003; Chen et al.,

2005; Wilen et al., 2008). Low dosages do not provide suffi-

cient charge neutralization/polymer bridging for flocculation

to be efficient, whereas very high concentrations lead to

deflocculation due to charge inversion and/or steric hin-

drance, which demonstrates that an optimum dosage of 

flocculants exists (Abo-Orf and Dentel, 1997; Poon and Chu,

1999; Lee and Liu, 2000; Yen et al., 2002; Chu et al., 2003;

Chen et al., 2005). The optimum dosage of polyelectrolytes

increases with concentration of suspended materials and the

concentration of single cells and EPS (Tiravanti et al., 1985;

Mikkelsen and Keiding, 2001). The optimum dosage of 

cationic polymer for flocculation of municipal CAS sludge hasbeen reported to be in the range of 0.01e0.06 mg/g SS (Yen

et al., 2002; Chen et al., 2005). When colloidal material is

released from the flocs due to factors such as shear or

anaerobic conditions, higher dosages of polyelectrolytes are

required (Mikkelsen et al., 1996; Abu-Orf and Dentel, 1997). In

general, sludge with low dewaterability also requires a higher

dosage of polymers.

12. Conclusion

Great variation in sludge dewaterability is observed among 

wastewater treatment plants; hence the floc and sludge

properties have a high impact on the specific filtrate flow rate.

The best dewaterability is observed for sludge that contains

strong compact flocs and low concentrations of single cells as

well as dissolved EPS. This gives the best sedimentation in the

clarifier, the highest permeate flux in MBR systems, the

highest filterability(belt filters and sludge mineralization bed),

the best effluent quality (decanter centrifuges), and lowers the

Table 4 e  Link between sludge treatment and dewaterability.

Parameter Effect

Conductivity Changes in conductivity (high conductivity or dilution) lower specific flowrate

This can be a problem due to road salting, intrusion of sea water and some

industries

Water hardness High water hardness improves specific flow rate

Calcium carbonate can be added to improved dewaterability

pH High pH leads to floc disintegration, which lowers the specific flow rate

The water content in the formed filter cake may be lower if the pH value is

lowered.

Storage Anaerobic storage lowers specific flow rate

Tanks/pipes with anaerobic pockets are problematic

Addition of nitrate during storage or aeration can improve the filterability

Pumping Vigorous pumping lowers specific flow rate

Gentle pumping and mixing is recommended. Avoid sharp bends on pipes

Treatment system Conventional plant usually gives better sludge than membrane bioreactors

(MBR). Sludge from digesters is difficult to dewater.

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amount of chemicals required for sludge conditioning. High

water hardness improves dewaterability because calcium ions

improve floc strength and reduce the concentration of single

cells and EPS. Variations in conductivity, and particularly high

conductivity and pH, reduce dewaterability as flocs disinte-

grate. Thus road salt in the winter season, intrusion of sea

water and some types of industries can result in lower dew-

aterability. Anaerobic storage lowers dewaterability as flocsare disintegrated and the conductivity increases. Anaerobic

storage or tanks with anaerobic pockets are more problematic

than aerobic or anoxic storage.High shear in pumps and pipes

destroys the flocs and reduces dewaterability and should be

avoided. The physico-chemical properties of biological sludge

cakes govern the sludge dewaterability; hence, the filtration

processes of biological sludge should be improved by

improving sludge physico-chemical characteristics.

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